Abrasive belt for sanding steel



Sept. 12, 1944. L. A. HATCH ET AL ABRASIVE BELT FOR SANDING STEEL Filed Aug. 3, 1940 www@ @ver2/23715 W- arnqg Patented Sept. l2, 1944 ABRASIVE BELT FOB SANDING STEEL Lloyd A. Hatch and John Edmund Clarke, St. Paul, Minn., assignors to Minnesota Mining` a Manufacturing Company, St. Paul, Minn., a

corporation oi'Delaware Application August 3, 1940, Serial No. 351,259

(ci. .fu-iss) The present invention relates to abrasive articles and particularly to abrasive articles such as endless abrasive belts for use in sanding or polishing steel or other hard metals or the like.

Particularly, we have found marked advantages are gained by employing abrasive grits in-such articles, the abrasive grits having been treated as herein defined more in detail.

Our invention is particularly directed to abrasive articles of the type and for the purpose mentioned which employs heat treated aluminum oxide abrasive particles or grits.

Two types of abrasive grits have been generally employed in the sanding of steel and other hard metals or similar materials; natural emery and,

aluminum oxide. Silicon carbide is, by nature, more brittle, or less tough, and consequently less suitable. Even after giving silicon carbide certain treatments in an endeavor to toughen the same, we have still found that it is commonly,-

less suitable than aluminum oxide.

Brieily, the abrasive we employ in our above referred to abrasive belts for sanding hard metals comprises aluminum oxide which has been heated above approximately 1000 C. after the same has been crushed to size. The heat treatment is normally within the range of 1000 C. to 1350" C. and is ordinarily preferably within the range of 1250 to 1300 C. The desired toughening action occurs when the aluminum oxide particles, crushed to size, are maintained within the temperature range above set forth for a period of, say, a few minutes to about a half hour, more or less.

Toughness ot abrasive grits may be measured by placing a. definitely sized sample in a predetermined ball mill with a predetermined charge of steel balls, and after rotation of the mill for a predetermined time, i. e. about 10 minutes, for example, removing the abrasive grits and again screening them to determine the percentage of grit that has resisted breakdown. Taking the toughness figure for raw crushed aluminum oxide grits as 100% and assuming, for example, that 40% of the raw crushed aluminum oxide grits would resist breakdown in such a ball mill toughness test, then when a heat treated aluminum oxide abrasive of similar size is placed in the ball mill and given a similar toughness test if, for example, 55% of the material is shown to have resisted breakdown (when subsequently screened), its toughness may be expressed as S/40X 100 or 137.5%. This, in fact, shows about the toughness of most heat treated aluminum oxide prepared according to the present invenss tion, as compared with the toughness of raw crushed aluminum oxide. However, by careful control of the 4heat treatment of the aluminum oxide we have ,been able to secure abrasive grits having a toughness of 150% and can fairly read-v ily, without too accurate control, secure a'. toughness of 140 to 145%. Crushed aluminum oxide of desired size should preferablyl be treated at a temperature-of ,about 1300 C., or closely approaching 1300" C. to secure best toughness.

While various.' toughness tests can be devised which will show comparative toughness oi.' heat treated anduntr'eated abrasive particles, after considerable experimentation we concluded that the following test gave about the best and fairest test of toughness: 25- grams of aluminum oxide abrasive grita-crushedto size,- and carefully screen to size, are introduced into a (6 inch jar) vball mill and thentwenty inch steel balls are introduced therein and-the' mill is rotated at 260-270 R. P. M. vfor 10 minutes. The abrasive particles are then removed and again re-sized using the same screen that was used in sizing them'originally. The numberof grams which resisted any substantial breakdown, divided bythe number of grams of Vraw aluminum oxide of the same size which resisted breakdown in the same test, times gives the toughness ligure for the particular abrasive grit in percentage.

'I'he heat treatment maybe effected in any suitable apparatus. One method which we have employed successfully is to charge .a batch of suitable size, for example, about 600 or 700 lbs. into a rotary kiln or smeltlng furnace having a diameter of, for example, about 6 feet andheating the same, as by directvilring, until the temperature is brought up within the desired range as hereinabove set forth. This may require about 2 hours 4from the time the raw crushed sized mineral is charged to the kiln until the heat treatment has been completed and the mineral is ready to be discharged. This is a. batch process.

However, where the quantities of material to be treated are suiliciently large and/or suitable equipment is available, the abrasive grit may be heat treated in a continuous process. Likewise, an indirect tired furnace, such as a muiile furnace or the like,may be employed to eiect the desired heat treatment.

While our abrasive belts for sanding or polishing steel or the like may vary appreciably in construction, one suitable construction is to employ a paper or cloth backing having a glue binder coat which holds a layer of heat treated abrasive grits as above described to the backing. Such articles also commonly have a sandslzing coat which may be of glue, or of natural or synthetic resinous or adhesive materials.

For the sake o fjllustrating our present invention, and not limiting it, reference is made to the appended drawing, in which:

Figure 1 is an enlagd diagrammatic illustration in section of a piece of untreated cloth backing,

Figure 2 is an enlarged diagrammatic illustration in section of a piece of impregnated cloth backing.

Figure 3 is an enlarged diagrammatic illustration in section of a finished abrasive belt.

Figure 4 shows, diagrammatically, an example of a belty made according to the present invention and how it may be used in a sanding machine.

Figure 3 shows the cloth backing It impregnated by treatment with an impregnating or presizing coat II. To this impregnated cloth the binder or grit-bonding coat I2 is applied. The abrasive grits Il are shown embedded in this binder coat I2 and the sandsize coat Il is shown overlying the binder coat I2, between the grits I3.

In Figure 4 the completed belt II is shown with the abrasive coating on its outward side. Belts made according to our invention may be used`in a wide variety of machines that are designed to abrade with abrasive belts. For the sake of clarity and to provide a convenient means of illustrating a completed belt in a drawing, the belt in Figure 4 is shown mounted in a sanding machine that supports the belt by two pulleys which contact the belt on its non-abrasive or inwardside. abraded by the belt, and which is illustrated as a steel element being smoothed or abraded, is pressed against the moving belt at the point at which the belt is backed up by the shoe I1.

It is to be understood that the drawing is simply illustrative of one specific embodiment of the present invention, and is in no sense intended as a limitation.

Where our abrasive belt is to be'employed wet, it will be evident that the exposed adhesive or resinous material may not be glue, since glue is not waterproof. Under such circumstances, we may, for example, give the cloth or paper backing a presize or impregnating coat and/or a backsize coat of vinyl resins such as vinyl acetals or butyrals, or of a material such as ethyl cellulose, or an olefine polysulphide resin such as Thiokb or other suitable materials. Over this may be applied a hard bond such as al suitable heat convertible phenolic resin. Various phenolic resins suitable for this purpose are well-known to 'those sinnedl in the art and are disclosed in various patents and pending applications.

Abrasive belts or the like so made, employing heat treated aluminum oxide as above set forth,

have an effective life in commercial use in polishing or abrading steel or other hard metals or the like much greater than when untreated aluminum oxide is employed; in fact the effective life in certain cases is approximately doubled and it is common for there to be upwards of a 25 to 40% improvement in effective life.

superatmospheric temperatures. Other abrasive materials when subjected to superatmospheric temperatures are not signicantly influenced with reference to their ability to be used in abrading steel or other hardmetals or the like. For example, flint abrasives are not improved by heat treatment and particularly not by the employment of temperatures hereinabove set forth.

Moderate heat treatments for the removal of organic materials from aluminum oxide, which we have heretofore employed, involve the use of temperatures up to about 600 C. No signicant toughening of the aluminum omde, however, occurs at such a relatively low temperature and no appreciable increase in toughness is noted on ordinary samples of aluminum oxide until a temperature of 1000 C. is reached or at least approximated. In tests which we have made, employing the ball mill test for toughness, no perceptible difference in toughness was noted between raw crushed aluminum oxide and crushed aluminum oxide heated to 600 C.; both showed the same approximate toughness figure.

While not limited thereto, we nd special advantage in using our heat treated aluminum oxide abrasive grits in a flexible abrasive belt The article I0 which is to be having a hard, tough, grit-bonding or binder coat such as a heat convertible phenolic or phenol-aldehyde resin, urea-aldehyde resins or preferably a butyl alcohol solution of urea-aldehyde resin plasticized with an air-drying or drying oil acid modified alkyd resin, or the like. Hence, a special feature of our invention involves employing our heat treated aluminum oxide abrasive particles in a hard, heat convertible synthetic vresin in making flexible abrasive belt, which may be used either wet or dry, for sanding and polishing steel or the like.

f In the sanding of steel with abrasive belts as herein described, we have found that a speed in the range of 2,000 to 7,000 surface feet per minute is desirable. More particularly we have found that in the sanding of steel a speed of the order of 2,500 to 3,500 surface feet per minute is an advantage with our abrasive article over the much higher speeds previously tried. These speeds have special application to the type of flexible abrasive articles referred'to in the last preceding paragraph.

What we claim is:

1. An endless abrasive belt for sanding hard metals and the like comprising a flexible backing, a coating of a bond thereon and embedded in said bond al layer of aluminum oxide abrasive grains of commercial grit sizes which have been heat treated at a temperature within the range of 1250 to 1350" C. f or a period of time sufllcient to increase the toughness thereof.

2. An endless abrasive belt for sanding hard metals and the like comprising a flexible backing, a coating of a bond thereon and embedded in said bond a layer of aluminum oxide abrasive grains of commercial grit sizes which have been heat treated at a temperature within the range of 1250 to 13001 C. for a period of time suillcient to increase the toughness thereof.

3. An endless abrasive belt for sanding hard metals and the like comprising a flexible backing, -a coating of a bond thereon and embedded in said bond a layer of aluminum oxide abrasive grains of commercial grit sizes which have been heat treated after crushing to size at a temperature of approximately l300 C. for a period of time suiilcient to increase the toughness thereof.

4. For use in a flexible abrasive belt for sanding hard metals including steel, aluminum oxide abrasive grains of commercial grit sizes heat treated at 1250 C. to 1350 C. for a period of suiiicient time to increase the toughness thereof.

5. An abrasive strip adapted to have its ends joined together as a belt for the high-speed sanding of hard metals and the like comprising a fibrous backing impregnated with a water-resistant coating comprising a thermoplastic synthetic resin, and overlying grit bonding or binder coat comprising a hard heat-convertible 'synthetic resin and, embedded in said binder coat, a

layer of aluminum oxide abrasive grains of commercial grit sizes which have been heat treated at a temperature of approximately 1250 to 1350 C. for a period of time suicient to increase the toughness thereof, said abrasive grains having a superior toughness, as measured by the ball mill test herein defined, and a substantially increased utility for Ithe purpose stated as the result of such heat treatment. v

LLOYD A. HATCH. JOHN EDMUND CLARKE. 

